The correct answer is C) A quick flashing red light at each lower corner. The Coast Guard regulations require that any obstruction or reduced clearance under a bridge over a navigable waterway be clearly marked at night with quick flashing red lights at the lower corners of the obstruction. This helps alert mariners to the reduced clearance and allows them to safely navigate around the obstacle. The other options are incorrect because: A) A strobe light would not be the appropriate lighting; B) Fixed amber lights would not provide the necessary visual cue for the reduced clearance; and D) Flood lights alone would not clearly indicate the specific lower corners of the obstruction.